M2 Wind-Driven Circulation

Topic

Surface ocean circulation is dominated by the wind, which exerts a drag force—known as wind stress—on the sea surface; it is precisely this stress that sets in motion the large subtropical and subpolar gyres dominating circulation across all oceans.

Sverdrup's theory links vertical Ekman pumping—which occurs when surface Ekman transport diverges or converges—to the vorticity imparted to the ocean by the wind field itself; this relationship means that, within the ocean interior, meridional water transport is determined entirely by wind vorticity, without requiring any knowledge of the water's stratification.

The intensification of western boundary currents—such as the Gulf Stream, the Kuroshio Current, the Brazil Current, the Agulhas Current, and the Malvinas Current—is a direct consequence of how the Coriolis parameter varies with latitude; this variation renders the Sverdrup solution unable to satisfy the boundary condition imposed by the western coast of each ocean basin without the presence of an intense, narrow western boundary current to close the gyre.

The Antarctic Circumpolar Current is the only major ocean current that flows around the entire globe without any continent obstructing its path; the absence of such continental barriers, combined with the constant drive of westerly winds, prevents this current from closing into a Sverdrup gyre, unlike the subtropical gyres described earlier.

The expected behavior of these gyres includes, first, large anticyclonic subtropical gyres that circulate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, collectively transporting between thirty and one hundred million cubic meters of water per second; That same transport is distributed very unevenly between a narrow, fast western boundary current—barely a hundred kilometers wide with speeds of one to two meters per second—and a much slower, broader interior return flow, spanning about a thousand kilometers with speeds of just one-hundredth to one-tenth of a meter per second. It is precisely Ekman pumping that causes water to sink—at a rate of merely one-millionth of a meter per second—in the centers of these anticyclonic gyres, whereas that same pumping drives upwelling at the latitudes where subpolar cyclonic gyres are located.

Thus, from the wind stress that opens this document to the subpolar upwelling that concludes it—taking in the Sverdrup balance, the intensification of western boundary currents, and the Antarctic Circumpolar Current, which defies that very balance—the wind organizes surface ocean circulation into a hierarchy of gyres: fast and narrow near the coasts, and slow and broad within the interior of each basin.

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